1. Field of the Invention
The present invention is directed to measurement of the refractive error in the eye, more particularly to methods and techniques for compiling a topographic mapping of these refractive errors.
2. Description of Related Art
Measurements of aberrations in an eye are important for diagnosis of visual defects and assessment of acuity. These measurements and their accuracy become increasingly important in light of the growing number of ways, both surgical and non-surgical, that aberrations can be corrected. These corrections rely on accurate, precise measurements of the entire ocular system, allowing successful screening, treatment and follow-up. Enhancements in the accuracy of ocular measurements may aid in improving the identification of patients in need of correction and the performance of the correction itself.
There are a number of current methods used to measure performance of the ocular system. The most widely used and well established are psycho-physical methods, i.e., methods relying on subjective patient feedback. The oldest of the psycho-physical methods is the foreopter or trial lens method, which relies on trial and error to determine the required correction. There are psycho-physical methods for measuring visual acuity, ocular modulation transfer function, contrast sensitivity and other parameters of interest.
In addition to these subjective methods, there are also objective methods for assessing the performance of the ocular system. Such objective methods include corneal topography, wavefront aberrometry, corneal interferometry, and auto-refraction. Many of these methods only measure the contribution of specific elements to the total refractive error. For example, much work has been directed to measuring the topography of the cornea and characterizing the corneal layer. However, the corneal shape only contributes about 30-40% of the total refractive error in most cases. In order to measure the bulk of the refractive error and to provide a complete mapping for diagnosis and correction, additional information and measurements are needed.
Another method for determining the refraction of the eye is auto-refraction, which uses a variety of techniques to automatically determine the required corrective prescription. These automated techniques include projecting one or more spots or patterns onto the retina, automatically adjusting optical elements in the auto-refractor until the desired response is achieved, and determining the required correction from this adjustment. However, auto-refractors are not considered especially reliable. Further, auto-refractors measure only lower order components of the aberrations, e.g., focus and astigmatic errors.
Recently, the eye has started being considered as an optical system, leading to the application of methods previously used for other optical systems to the measurement of the eye. These methods include interferometry and Shack-Hartmann wavefront sensing. These techniques are of particular interest because they measure the complete aberrations of the eye. This additional information allows measurement of non-uniform, asymmetric errors that may be affecting vision. Further, this information may be linked with any of the various corrective techniques to provide improved vision. For example, U.S. Pat. No. 5,777,719 to Williams describes the application of Shack-Hartmann wavefront sensing and adaptive optics for correcting ocular aberrations to make a super-resolution retina-scope. U.S. Pat. No. 5,949,521 to Williams et al. describes using this information to make better contacts, intra-ocular lenses and other optical elements.
Wavefront aberrometry measures the full, end-to-end aberrations through the entire optics of the eye. In these measurements, a spot is projected onto the retina, and the resulting returned light is measured with an optical system, thus obtaining a full, integrated, line-of-sight measurement of the eye""s aberrations. A key limitation of the instruments used in these measurements is the total resolution, which is ultimately limited by the lenslet array of the instrument. However, selection of the lenslet array is itself limited by several factors, most importantly the size of the spot projected onto the retina.
A schematic illustration of the basic elements of a two dimensional embodiment of a Shack-Hartmann wavefront sensor is shown in FIG. 2. A portion of an incoming wavefront 110 from the retina is incident on a two-dimensional lenslet array 112. The lenslet array 112 dissects the incoming wavefront 110 into a number of small samples. The smaller the lenslet, the higher the spatial resolution of the sensor. However, the spot size from a small lenslet, due to diffraction effects, limits the focal length that may be used, which in turn leads to lower sensitivity. Thus, these two parameters must be balanced in accordance with desired measurement performance.
Mathematically, the image on the detector plane 114 consists of a pattern of focal spots 116 with regular spacing d created with lenslets 112 of focal length f, as shown in FIG. 3. These spots must be distinct and separate, i.e., they must be readily identifiable. Thus, the spot size xcfx81 cannot exceed xc2xd of the separation of the spots. The spot separation parameter NFR can be used to characterize the lenslet array 12 and is given by:                               N          FR                =                  d          ρ                                    (        1        )            
The relationship between the size of a lens and the focal spot it creates, where xcex is the wavelength of the light, is given by:                     ρ        =                  1.22          ⁢          f          ⁢                      xe2x80x83                    ⁢                      λ            d                                              (        2        )            
for a round lens or                     ρ        =                              f            ⁢                          xe2x80x83                        ⁢            λ                    d                                    (        3        )            
for a square lens. Thus, for a square lens, the separation parameter can be given by:                               N          FR                =                              d            2                                f            ⁢                          xe2x80x83                        ⁢            λ                                              (        4        )            
This is also known as the Fresnel number of the lenslet. To avoid overlapping focal spots, NFR greater than 2. In practice, the Fresnel number must be somewhat greater than two to allow for a certain dynamic range of the instrument. The dynamic range of a Shack-Hartmann wavefront sensor can be defined as the limiting travel of the focal spot such that the edge of the spot just touches the projected lenslet boundary, given by:                               θ          max                =                                                            d                2                            -              ρ                        f                    ⁢                      xe2x80x83                    ⁢          or                                    (        5        )                                          θ          max                =                                            d                              2                ⁢                f                                      -                          λ              d                                =                                    [                                                                    N                    FR                                    2                                -                1                            ]                        ⁢                          λ              d                                                          (        6        )            
Thus, the dynamic range is directly proportional to the separation parameter and the lenslet size.
A particularly useful arrangement for a Shack-Hartmann wavefront sensor ocular measuring system places the lenslet array in an image relay optical system at a plane conjugate to the pupil or corneal surface. In this configuration, the spot size on the detector of the wavefront sensor is given by:                               ρ          2                =                              1            M                    ⁢                                    f              L                                      f              e                                ⁢                      ρ            1                                              (        7        )            
where M is the magnification of the imaging optics, fL is the focal length of the lenslet array, fe is the focal length of the eye and xcfx811 is the spot size on the retina.
Comparing Equations (5) and (7), it is evident that the dynamic range of the wavefront sensor is limited by the size of the spot xcfx811 projected on the retina. For a practical system, the dynamic range must be able to resolve errors in the optical systems. Thus, the dynamic range is a key limited parameter of the entire system design. In previous implementations of the Shack-Hartmann wavefront sensor used for ocular measurement, the dynamic range has been increased by increasing the size of each lenslet. However, the eye itself can have significant aberrations. Thus, any beam projected into the eye will become aberrated, spreading the focal spot and increasing the spot size xcfx811 on the retina.
Various techniques have been implemented to address this problem. A small diameter beam has been used so that the total wavefront error is minimized across the injected beam. Another proposed solution projects the light into the eye at the focal point of a long focal length lens, operating as a field lens so that the size of the focal spot is not affected by the eye aberrations. In practice, for both of these cases, the beam is still somewhat large and is increased in size by the aberrations of the ocular system.
Another limitation on the dynamic range of the system is the sampling size. With a large spot on the retina, the sample size of the wavefront sensor must be increased to allow even a minimal dynamic range to be realized. For ocular systems with strong aberrations, such as found in people with large astigmatism or for those having undergone LASIK, the aberrations over each lenslet are sufficient to degrade the lenslet focal spot. Thus, the system is limited not just by focal spot overlap, but by the fact that the focal spots themselves fade out or are difficult to track. Using a small sample size does not allow sufficient light to be gathered, since the light is scattered by the retina into a large number of focal spots. Due to safety considerations, the input power may not be increased to compensate for this scattering.
The present invention is therefore directed to measurement of refractive errors of an eye that substantially overcomes one or more of the problems due to the limitations and disadvantages of measurements of the related art.
It is an object of the present invention to measure the end-to-end aberrations of the eye with sufficient accuracy and dynamic range in a practical manner.
It is a further object of the present invention to project a light beam into an ocular system so as to minimize the size of the focal spot on the retina.
It is another object of the present invention to use this smaller focal spot to allow much greater sampling density of the ocular system, thereby enhancing the accuracy and dynamic range.
It is yet another object of the present invention to make a practical, low cost system, available for use in a clinical setting.
At least one of the above and other objects may be realized by providing a system for measuring errors in an eye including a projecting optical system which delivers light onto a retina of the eye, a pre-correction system which compensates a light beam to be injected into the eye for aberrations in the eye, the pre-correction system being positioned in between the projecting optical system and the eye, an imaging system which collects light scattered by the retina, and a detector receiving light from the retina collected by the imaging system.
The detector may be a Shack-Hartmann wavefront sensor, a shearing interferometer, a Moire deflectometer, or other passive phase measurement systems. The pre-correction system may include a telescope having at least one movable lens, fixed lenses inserted at an intermediate image plane, adaptive optical elements, and/or a cylindrical telescope. The pre-correction system may correct for focus and/or astigmatism errors in the eye. The telescope may be arranged so that a fixed lens of the telescope is one focal length away from the eye. Components used in the pre-correction system may also be used in the imaging system.
The pre-correction system may include a feedback loop which determines an appropriate pre-correction to be supplied by the pre-correction system. The feedback loop may include a detector receiving light returned from the retina, a processor comparing detected light with a desired feature of the light and adjusting at least one parameter of the pre-correction system in accordance with the comparison. The feedback loop may further include a return optical system for gathering the light from the retina. The return optical system may include the pre-correction system. The desired feature may be a minimized spot size on the retina.
The system may include an aperture that limits the angular dynamic range of the system. The system may further include a polarizing beam splitter between the eye and the wavefront sensor. The system may include an aligner that determines an appropriate eye alignment of the system. The projecting optical system may provide light to the eye at an angle to a central axis of the eye. The system may include an additional optical system between the detector and the eye. The system may include a power monitor which monitors power of the light beam being injected in the eye. The system may include an eye position detection system including a target projected on the eye, a position detector sensing the eye, and an adjustment system which adjusts a position of the system relative to the eye until the eye is in focus on the detector.
These and other objects of the present invention will become more readily apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating the preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.